Troops
125 - Guardian Defenders x 10 + Scatter Laser + Warlock + Embolden
150 - Waveserpent + Star Cannon + Spirit Stones + Star Engines
These guys are alright, though I'd strongly consider giving them a singing spear and destructor rather than embolden. Star Engines are nice but not a necessity. These changes net +7pts.
135 - Guardian Storm Squad x 10 + Fusion Gun x 2 + Warlock + Singing Spear + Enhance
160 - Waveserpent + Bright Lance + Spirit Stones + Star Engines
I don't think I'd bother with enhance on these guys, unless you were adding an Autarch to the unit or had something else devious in mind. Since they seem to be tank hunters I'd consider embolden on this team; the farseer could ride with them if he remains on foot. Since you're using star cannons on the other serpent, I'd put them here too. Star Engines - again, not so strongly needed since this unit is primarily gonna be hunting tanks and therefore rarely moving over 12 [also to fire the starcannons it can't move over 12]. Cost savings is now up to 37pts.
162 - Dire Avengers x 10 + Exarch + Power Weapon and Shimmer Shield + Defend
135 - Waveserpent + Shuriken Cannons + Spirit Stones + Star Engines + Shuriken Cannon
Agreed that in this case, if the Avengers are primarily gonna be shooting, and not the recipient of fortune, you're better off with dual cats and bladestorm. I'd actually keep the star engines on this serpent, cost savings up to 47pts.
207 - Guardian Jetbikes x 6 + Shuriken Cannon x 2 + Warlock + Destructor
These guys ought to get embolden, you want to avoid the front till absolutely necessary and really want this resilient unit to benefit from fortune and ld rerolls. Cost savings up to 52pts.
168 - Ranger Pathfinders x 7
I'd consider dropping down to 5 or going up to 9, mostly due to cost vs leadership for 25% casualties issues. And if you aren't considering outflanking them, performing first turn assaults, firing pistols and etc, and especially if your farseer will be elsewhere [and he probably ought to be], try dropping them down to 5. Cost savings up to 100pts.
HQ
120 - Farseer + Guide + Doom + Spirit Stones
This guy really wants fortune for the jetbikes - if he's on a jetbike he can hang with the pathfinders for guide early on, then once the enemy closes advance to whip catapult doom on the enemy. Not having a spear is a terrible waste of one of the highest BS values available and would be a nice addition. If he's not on a jetbike himself, guide may not make a whole lot of sense [and fortune would make more sense], since the best places for him will be in transports with guardians. Since he'd have good synergy with the stormguard in a tankhunting role [and be forward enough to doom just about anything], I'd consider swapping guide for fortune, and keeping the jetbikes up front and nearby as a baseline for what this guy ought to do. Cost savings down to 87pts.
Fast Attack
130 - Vyper + Shuriken Cannon + Star Engines + Holo Field + Spirit Stones + Shuriken Cannon
From personal experience! the holofields are a bad idea on vypers, especially since they are destroyed on a 3+ on penetrating hits. The star engines are not so hot on a unit that's not likely to live long enough to use them, and that would prefer using its short lifespan to shoot up enemies. Also, you've costed it wrong. With correct cost and removal of superfluous upgrades, cost savings become 147pts.
Heavy Support
175 - Dark Reapers x 5
This will work nicely as is.
130 - Wraithlord + Wraithsword + Star Cannon
This can probably remain as is, as well.
Elites
80 - Fire Dragons x 5
I'd advocate removing these guys. You've plenty of AT options in all other areas of your FOC. Cost savings up to 227pts!
123 - Howling Banshees x 6 + Exarch + Executioner + War Shout
With those 227pts I'd think about increasing the size of the Howling Banshees and adding a waveserpent to carry them. Bringing the unit to full strength and adding one of these:
Waveserpent + Star Cannon + Spirit Stones
adds 183pts to the list, leaving 44pts. At this point you'd have enough pts to add a jetbike to the farseer if desired, or bump the pathfinders back up to 6 and add runes of warding to the farseer.
I'd imagine such an army would function like so:
Guardian defenders - transport hunters
stormguard [farseer if no bike added] - AT
jetbikes [farseer if bike added] - AT
pathfinders, serpents, reapers, wraithlord - heavy infantry and monstrous creature suppressors
Avengers - used to eliminate withered units
Banshees - remain in serpent in position to assault enemies that get too close to your troops as they deploy, or to crush enemy units near the flanks.
Four starcannons [TL or BS4] plus the pathfinders plus the reapers will answer for many an armored infantryman, and you've got plenty to deal with hordes. Should be able to reliably dump about 8 wounds on a unit of choice in cover, or 10 wounds across 2 units not in cover.